A positive displacement pump makes a fluid move by trapping a fixed amount and forcing (displacing) that trapped volume in to the discharge pipe. 
… Liquid flows into the pumpas the cavity on the suction side expands and the liquid flows out of the discharge since the cavity decreases.
The most common type among centrifugal pump is the radial flow pump. … The main difference between these types of pumps and centrifugal can be that positive displacement pumps will move liquid at the same quickness regardless of the strain on the inlet end and centrifugal pumps won’t